Conventional cylinder lawn mowers usually comprise a cylinder unit with a plurality of peripherally mounted helical knives, which cooperate with an abutment arranged at the underside of the cylinder unit.
When cutting grass by means of such a cylinder lawn mower, the vegetation on a lawn, hereinafter referred to as grass, is inserted between the respective knives and the abutment. The cooperation of the knives and the abutment yields an almost scissor-like cutting of the grass, which is preferable since it results in a distinct cutting surface which thus is kind to the grass.
Nevertheless, cylinder lawn mowers of the above-mentioned type suffer from certain drawbacks.
When the grass is long, the blades of grass may intersect and press each other down. The grass may also be flattened for some other reason, for example, due to precipitation or if someone has trampled it down. Owing to this, the grass may not be properly inserted between the abutment and the respective knives. It goes without saying that this will result in the grass not being cut at all or not being cut in a satisfactory manner. Another reason for the grass not being cut satisfactorily, in particular in the case of motor-driven lawn mowers, is that the air stream generated by the cylinder presses the grass down so that it will not contact the abutment and the knife.
Occasionally, individual blades of grass or plants (such as dandelions), hereinafter generally referred to as blades of grass, may shoot up from the lawn. These long blades of grass are usually difficult to cut using a cylinder lawn mower since they are almost always pushed forward and then bent under the abutment, thereby not being cut. As a result, extra work is needed to obtain a satisfactory cutting result.
Another problem relates to mulching, i.e. cutting of the clippings into finer pieces so that they can be returned to the lawn in an aesthetically attractive manner.
A conventional cylinder lawn mower cuts grass at a preset cutting level. The cut grass is then usually spread on the new-cut lawn and remains clearly visible, often in an unattractive way. This is why the grass is often collected and removed from the lawn. The cut grass can, for example, be collected by being put directly in a container or by being raked up after the cutting of the lawn. However, this is unfavourable since nutrients are removed from the lawn.
In rotor lawn mowers, this problem is partly solved by means of a system for cutting the grass into finer pieces. Thanks to this, the grass can be returned to the lawn in an aesthetically attractive manner since the finely cut grass can filter back into the lawn, thereby being less conspicuous. Furthermore, the clippings decompose more quickly if cut into finer pieces. One type of such a rotor lawn mower retains the clippings in the rotor housing for further cutting into finer pieces, whereupon they fall down on the lawn.
Mulching is thus common today in rotor lawn mowers, but it has been found that it is difficult to provide mulching in cylinder lawn mowers, especially as cylinder lawn mowers are known to cut each blade of grass once at the lowest level without any further processing of the clippings.
One type of cylinder lawn mower is known from U.S. Pat. No. 2,157,620. This cylinder lawn mower comprises a rake-like means which is arranged in front of the cylinder unit to force the grass into an upright position before it is cut. This cylinder lawn mower solves the problem of flattened grass. It does not, however, solve the problems involved in the cutting of long grass or individual long blades of grass. This prior-art cylinder lawn mower, if anything, makes these problems even worse since instead of raising the blades of grass to facilitate the cutting thereof it bends them in the travelling direction of the cylinder lawn mower. As a result, the blades of grass end up under the abutment and are thus not cut. In addition, the prior-art cylinder lawn mower does not offer any solution to the problem of mulching, i.e. the cutting of the clippings into finer pieces.
Different types of cylinder lawn mowers have been suggested to solve at least one of the above-mentioned problems. In particular, cylinder lawn mowers of the type stated by way of introduction are known, which comprise insertion means for engaging and inserting grass into the abutment.
Such a cylinder lawn mower, comprising teeth arranged on the knives, is disclosed in U.S. Pat. No. 1,109,776. In U.S. Pat. No. 1,970,489, a cylinder lawn mower is described which comprises bars which are provided with teeth and which are arranged between the knives. These prior-art cylinder lawn mower do not, however, solve the above-mentioned problems since the teeth on the bar-like structures contribute to the compression of the grass, which yields an unsatisfactory cutting result.
Another variant of a cylinder lawn mower is disclosed in U.S. Pat. No. 2,056,369. Also in this document, the cylinder unit comprises bars provided with teeth. However, the bar provided with teeth is guided by a groove in an oval path to engage grass outside the radius of gyration of the knives. This does not either solve the above-mentioned problems since the bars provided with teeth press down the grass.
U.S. Pat. No. 2,730,857 discloses another variant of a cylinder lawn mower, which comprises pins which are supported by the same structure as the knives. The pins extend outwards to the same radial level as the knives or somewhat inwardly thereof. Also this variant fails to solve the above-mentioned problems, since the pins do not come into sufficient contact with the grass, thereby not providing any additional insertion of grass between the knife and the abutment.
U.S. Pat. No. 3,144,745 describes a cylinder lawn mower in which teeth are arranged on the knives. The teeth extend beyond the knives in the radial direction, thereby providing improved engagement with the grass. The abutment has recesses matching the shape of the teeth, so that the teeth will not hit the abutment as they pass. However, this solution is not satisfactory as the teeth will engage the grass and feed it to a position under the abutment where the knives cannot cooperate with the abutment for cutting.